Chapter 9 Soil

Chapter 9, Soil | Class 7 |
Science| NCERT Solutions

Tick the most suitable answer in questions 1 and 2.

Question 1. In addition to the rock particles, the soil contains

(i) air and water

(ii) water and plants

(iii) minerals, organic matter, air and water

(iv) water, air and plants

Answer 1 (iii) minerals, organic matter, air and water.

Question 2. The water holding capacity is the highest in

(i) sandy soil

(ii) clayey soil

(iii) loamy soil

(iv) mixture of sand and loam

Answer 2 (ii) clayey soil

Question 3. Match the items in Column I with those in Column II:

          Column I                                            Column II

(i) A home for living organisms             (a) Large particles

(ii) Upper layer of the soil                       (b) All kinds of soil

(iii) Sandy soil                                           (c) Dark in colour

(iv) Middle layer of the soil                    (d) Small particles and packed tight

(v) Clayey soil                                          (e) Lesser amount of humus

Answer 3

             Column I                                        Column II

(i) A home for living organisms             (a) All kinds of soil

(ii) Upper layer of the soil                       (b) Dark in colour

(iii) Sandy soil                                           (c) Large particles

(iv) Middle layer of the soil                    (d) Lesser amount of humus

(v) Clayey soil                                           (e) Small particles and packed tight

Question 4. Explain how soil is formed.

Answer 4

Soil is formed by weathering. Breaking of bigger rocks into fine smaller mineral particles is called weathering. This is done in 3 ways:

1)Physical
2)Chemical
3)Biological

Physical (Sun, water, wind etc.)

Sun : It heats up rocks during the day so that they expand. At night these rocks cool down and contract. Since all parts of rock do not expand and contract at same rate, this result in the formation of cracks and ultimately the huge rock breaks up into smaller pieces.

Water : It could get into the cracks in the rocks formed due to the uneven heating by the sun. If this water later freezes, it would cause cracks to widen. Flowing water wears away even hard rock over long period of time. Fast flowing water often carries big and small particles of rock downstream. These rocks rub against other rocks and resultant abrasion causes the rocks to wear down into smaller particles. The water then takes these particles along with it and deposit it further down its path.

Wind : Strong wind also erode rocks down. The wind also carries sand from one place to the other.

Chemical
It involves a variety of chemical processes such as hydrolysis, hydration, oxidation, reduction.
It involves breaking down of complex compounds by carbonic acid present in water and by acidic substances derived from decomposition process of organic matter in soil.

Biological
Lichens, mosses and other plants live on rocks and produce acids, which accelerate the process of rock weathering.

Question 5. How is clayey soil useful for crops?

Answer 5 Clayey soil is useful for crops because:

1) It has very good water holding capacity.

2) It is rich in humus and is very fertile.

3) It contains many useful organic minerals.

Question 6. List the differences between clayey soil and sandy soil.

Answer 6

Clayey soil	Sandy soil
Particles are very small in size.	particles are quite large in size.
It can hold good amount of water.	Its ability to retain water is low.
It is rich in humus.	It is not rich in humus.
Less air is trapped between the particles.	More air is trapped between the particles.
Suitable for crops like paddy , wheat and gram.	Suitable for crops like peanuts, cotton

Question 7. Sketch the cross section of soil and label the various layers.

Answer 7

Question 8. Razia conducted an experiment in the field related to the rate of percolation. She observed that it took 40 min for 200 mL of water to percolate through the soil sample. Calculate the rate of percolation.

Answer 8 Amount of water taken = 200 mL

Percolation time = 40 min

 Question 9. Explain how soil pollution and soil erosion could be prevented.

Answer 9 Soil pollution can be prevented in following ways:

1) Industrial discharge should be treated before release.

2) Use manures instead of chemical fertilisers.

3) Pesticides and insecticides should used in limited amount.

Soil erosion can be prevented in following ways:

1) Methods like crop rotation and mixed farming should be encouraged.

2) More and more trees should be planted.

3) Deforestation should be reduced.

Question 10. Solve the following crossword puzzle with the clues given:

Across

2. Plantation prevents it.

5. Use should be banned to avoid soil pollution.

6. Type of soil used for making pottery.

7. Living organism in the soil.

Down

1. In desert soil erosion occurs through.

3. Clay and loam are suitable for cereals like.

4. This type of soil can hold very little water.

5. Collective name for layers of soil.

Answer 10